The present invention relates in general to fabrication of semiconductor devices and in particular to a system used for achieving alignment between a mask carrying a semiconductor pattern and a semiconductor wafer by using a linear Fresnel zone plate.
Conventionally, there is a system for achieving proper alignment between a mask carrying a desired semiconductor pattern and a semiconductor wafer by using a circular Fresnel zone plate (Fay, B. and Novak, W.T., "Automatic X-Ray Alignment System for Submicron VLSI Lithography. Solid State Technology, May, 1985). The circular Fresnel zone plate works similarly to a convex lens and focuses a radiation such as light or X-ray passing therethrough at a predetermined focal length as a result of diffraction caused at the edge between alternating transparent and opaque concentric rings constituting the plate. In this prior art system, a pair of circular Fresnel zone plates are provided on the mask and another circular Fresnel zone plate is provided on the wafer. In operation, they are illuminated obliquely from a top so that the radiation is incident to the wafer after passing through the mask. The radiation thus incident to the wafer is then reflected and passed through the mask in the opposite direction. At the time of reflection by the wafer and at the time of passing through the mask in the second time, the radiation is focused by the circular Fresnel zone plates on the wafer and on the mask. When the mask and the wafer are properly aligned, the spot of the radiation thus focused by these circular Fresnel zone plates are aligned on a straight line. Therefore, one can determine the state of alignment of the mask to the wafer by observing the relation between these three spots thus formed by suitable means such as a CCD device or camera.
In this prior art system, however, there is a problem in that a large area on the mask and the wafer are occupied by the circular Fresnel zone plates. In fact, at least a pair of such set of Fresnel zones, each including the three circular Fresnel zone plates therein, two of which on the mask and one on the wafer, have to be used in order to achieve a two dimensional alignment of the wafer and the mask. As such a set of Fresnel zone plates have to be provided on each of the masks, the area occupied by the circular Fresnel zone plates is further increased when a number of such masks are used for the patterning as is usually practised in the actual patterning of the semiconductor devices.
Alternatively it is proposed to use a linear Fresnel zone plate in the system for aligning the wafer and the mask. In this system, the Fresnel zone plate referred to hereinafter as LFZP is provided on the mask so as to focus a parallel beam of the radiation such that the radiation passed through the mask is focused on the wafer as a line-shaped image similarly to the case of a cylindrical lens. The wafer on the other hand carries a diffraction grating which may be a row of projections or depressions extending along the line-shaped image. Such a grating diffracts the radiation to a predetermined direction when irradiated by the focused radiation through the LFZP.
FIG. 1(A) shows a typical conventional LFZP comprising a plurality of alternating transparent and opaque bands arranged in one direction. Such a structure behaves similarly to the cylindrical lens as schematically illustrated in FIG. 1 (B). The LFZP is symmetrical about its central line 1 and there holds a relation: EQU r.sub.n.sup.2 =nf.lambda.+(n.lambda.).sup.2 /4 n=1,2, (1)
where r.sub.n stands for a distance measured from the central line 1 to a boundary between an nth band which may be opaque or transparent and an (n+1)th band which may be transparent or opaque, f stands for the focal length, and .lambda. stands for the wavelength of the radiation.
As a result, there occurs an interference of the diffracted light in a focal plane which is separated from the LFZP by the distance f as schematically illustrated by an image m which represents the distribution of intensity of the diffracted radiation on the focal plane. When such a diffracted radiation is incident to a diffraction grating, there occurs a diffraction in a predetermined direction.
FIG. 2 is a diagram showing a conventional system used for aligning a mask 10 and a wafer 11. Referring to the drawing, a radiation 12 produced by a radiation source 13 is focused on a first LFZP 14a having a first focal length via a mirror 15. The wafer 11 is provided with a diffraction grating (not shown) and is transported horizontally while maintaining proper focusing until a strong diffraction is observed by a detector (not shown) held with a predetermined angular relation with respect to the source 13. As a result, a coarse alignment of the mask and wafer is achieved. Next, both of the radiation source 13 and the mirror 15 are moved horizontally to a position indicated by a broken line. In this state, the radiation falls on a second LFZP 14b having a second focal length which is substantially shorter than the first focal length. At the same time, the wafer 11 is raised such that the radiation diffracted by the second LFZP 14b is properly focused on the wafer 11. In correspondence to the LFZP 14b, there is provided a second grating (not shown) on the wafer 11 such that there appears a strong diffraction to a predetermined direction when the radiation correctly falls on the second grating. In order to detect the properly aligned position, the wafer is moved to and fro horizontally until the detector detects the diffraction from the second grating. By doing so, a precise alignment between the mask and wafer is achieved. In a typical example, the LFZP 14a has a focal length of 50 .mu.m and the LFZP 14b has a focal length of 10 .mu.m.
In such a prior art construction, however, there is a problem in that one has to use at least two LFZPs in order to achieve the alignment in one direction such as X or Y (FIG. 2). In correspondence to this, two diffraction gratings must be provided on the surface of the wafer for each direction. As a plurality of masks are usually employed for the patterning in the actual semiconductor devices, the number of the LFZPs on the mask and the number of corresponding gratings on the wafer increases with a rate twice as large as the number of the masks. For example, ten LFZPs in total and ten corresponding gratings have to be provided on the mask and on the wafer when patterning a device comprising five semiconductor layers. Thus, substantial area on the wafer and the mask is occupied by the marks such as the LFZP and the grating for the alignment.
Further, one has to move the radiation source and the associated optical system such as the mirror 15 horizontally each time the coarse alignment is completed. Associated with the movement, there arises a problem of achieving proper alignment between the thus moved optical system and the mask.